rpmsg: glink: spss: Unmap mailbox descriptors correctly

The mailbox descriptors registers are mapped using the devm variant of
ioremap. Switch the unmap function to the devm variant to prevent unmap
warnings when the glink device is unregistered.

Change-Id: Iafe0fcb673f215479ca13b1e0df0d96bdbc87fc8
Signed-off-by: Chris Lew <clew@codeaurora.org>